I've thought about the idea of a sleeping bag snorkel. Some soft rubber device that one holds in the mouth and sticks out of the exit of the cinched down hood. Outgoing air would warm a felt or wool "heat exchanger" and then incoming air would be heated by the warm felt. This might also prevent icing of the felt or wool heat exchanger. This would allow sleeping with the head fully covered by the bag and being able to breath warmed, moistened air. It might also prevent wetting of the bag around the "blow hole". I've experienced breathing warm air just by using a wool hat pulled over my face.
Well, I thought this was a pretty weird idea until I found the following patent:
http://www.google.com/patents/about?id=xLIzAAAAEBAJ&dq=sleeping+bag+snorkel
Not really the same thing, but at least someone else had been thinking along these lines.
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